Thermionic valve apparatus



Nov. 20, 1945. R. FORTESCUE 2,389,145

THERMIONIC VALVE APPARATUS Filed Oct. 12, 1958 10 8 MOM/LA T50 IMPEDANCE m p'ar 0mm rnvs NETWORK lNPU 7" OF MOD (/LA 77 ON PR ODU C 7'5 P/PEOOM/NA TELY i UNMOD ULATED Fig; 2.

IVE TWORK 0 Patented Nov. 20, 1945! THERMIONIO VALVE APPARATUS Rlchard Lewis Fortescue, London S. W; 12, England, assignmto Standard Telephones and ables Limited, London, England, a British company, and Marconls Wireless Telegraph Company Limited, London, England, a British oom- Application October 12, 1938, Serial No. 231,535

In Great Britain 0mm 23, 1931 19 Claims. (Cl. 179-1715) The present invention relates to thermionic valve apparatus, and is concerned with apparatus which is required to deliver modulated alternat-f ing current power to a load.

The invention islparticularly applicable to the output stage of s v. .broadcast.or like transmitter using'low-powermodulation; and to, other appw,

ratus in which high efllciency,in terms of power conversion of direct current into alternating cur rent, is desired.

posed comprises two amplifying valves coupled to a common load-an difed with modulated carrier oscillations of the same Waveform, one of thetwo valveshaving its c'ontrolgiid biasd be yonci' anode-current, cuti-off so that itfis'onlyl operativewhen the amplitude of the, applied, driv-' ing oscillations, exceeds the amplitude of the carrier, in lts'iunmodulat'ed state, The other valve .1 A power am'plifierwliich. has already been prov fii Sens One of" the valvesfisfv coupled 1 directly Ito, i the load andfthejother i .jcoupiq on was @11 l eecemen when t a, fieruol thet have, anfeiliciency substantially greate oscillation," add load during half-periodsifiwhen the modulation amplitude is rising, andto absorb power from the load during half-periods of falling modulation amplitude. A I he aclditional valvegapparatus; is coupled to the-load-ithrongh an;;ini pedance-in-,, i

ve t s etw ndm ta ez e-mimsqit zv va- I a' modulated carrier oscillation compris n inano ca i m on ntf na i t aS de-b ni fedf;

sci lat on nv nt onYaims ltot wiill i i fi diil li i e s est eiiicient thermionic valve apparatus for;

1b '1 c. series or parallel to the load.

The present invention accordingly provides apjb pro o di tional' thermionic valve apparatusbei gprbvid u to supply further high-frequency power to the a from jcarri engthe modulation stance being infin o ing in their "inp ft Oll' Dflratus of the kind reierredto for causing a current corresponding to a modulated carrier oscillation to flow in a load. said apparatus comprise ing two thermionic valves coupled'tovthe load and means for applying to their inputcircuits, driving oscillations of diflerent characters, wherein both driving oscillations include a component of can rier frequency and whereinthe modulation depth 1 v of the driving oscillation fed to eachvalveiis different both from that of the driving oscillation fed to the other valve and. iroin that oi th e oscillation setup in the load.

For the, purpose of! this 'speciflcatiomthe, term by an unmodu lated carrier, whic have a .niodulation f depth or z'ero;

The invention also {p p i s-i= a d l'a p t e minnow flowvi ag Sa d w m ri n IW -lfih i valves coupled to the load andflmeans a e m ytbaiedtw th o.

a parat arrangement and to simplify power supply arrangements. The driving voltages may, of course,

be amplified before application to the two valves, the number of stages of amplification depending on practical requirements.

In the preferred arrangement according to the invention, one of two valves, both of which are coupled to the load, is fed with modulated carrier oscillations in which there is a preponderance of the carrier component, and the other valve is fed with oscillations having a preponderance of the side-band components; the efficiency or the first valve may then be made to approximate that of a continuous wave amplifier, and the other can be made to consume negligible current in the unmodulated carrier condition.

If the required voltage wave form at the load is, for example VU-i-flt) 1 sin at where a(t) is a complex quantity representing a complex modulating wave including components which modulate the carrier to different depths, and u is the carrier frequency multiplied by 21, then the driving voltages applied to the'grids of the two valves may be expressed, respectively, by

where a and I! depend upon valve characteristics and circuit constants, and upon the requirements in respect of rectilinearity (with respect to the modulation envelope) and efficiency. The phase diii'erence between these two modulated carriers is necessary in order to compensate for the phase shift of 90 introduced by the impedance-inverting network by which one valve is coupled to the load. The values of a and p are worked out for a special case, by way of example, hereinafter. It is to be noted that although the invention includes the cases in which either a or p is zero, it does not include the case in which both are zero.

In a preferred arrangement, one valve is fed with substantially unmodulated oscillations of the carrier frequency. and is operated so that it has a low apparent internal impedance but is subject to a nearly constant alternating anode voltage. so that it functions as a continuouswave amplifier with a very high eiilciency. The other valve, in the preferred arrangement, is fed with oscillations representing the side-bands of the oscillations which are tobe set up in the load. together with a carrier-frequency oscillation of such amplitude that the first-mentioned valve is prevented from driving carrier-frequency current through the second valve. It will be noted that thisistheeaseinwhichaisaermunderthese conditions, B is generall small.

The application of the invention to the output stage-of a broadcast transmitter will now be described by way of example with reference to the accomp nyin diagrammatic drawing. In this application of the invention, the load to which modulated carrier power is to be supplied, is the aerial system.

l lgureslandzillustratetwoembodimentsof the invention, and Fig. 1a illustrates a modification of Pig. 1, while Fig. 1b shows by label a variation of input content as applied to tube I. Thesamereferencesareemplwedforlikeparts ofallthefism'cs. InFig. itwovalvesareconneetedinserieswithaloadandinl'igItwo valves are connected in parallel with the load.

Referring to Hg. 1, an output stage comprises two thermionic valves I and I, both valves being rent to fiow. Sources of bias potential, I and I.

are provided for this purpose.

The valve I, which will be referred to for convenience as the carrier valve, has its control-grid coupled b means of a transformer I to a source (not shown) of unmodulated carrier oscillations. and its anode is connected through a choke coil 6 to a source (not shown) of anode voltage. The anode of the valve I is also connected through a variable condenser I, an inductance coil I and the load I, all in series, to earth, and the cathode of the valve is also earthed. The load I is shown diagrammatically as a resistance. The valve I has its control grid coupled by means of a transformer II to a source (not shown) of driving oscillations, the nature of which will be further discussed below, The anode of the valve 2, which will be referred to for convenience as the side-band valve, is connected through an inductance coil II to the source of anode voltage, and through a condenser II and an inductance coil II in series to earth, its cathode being also earthed. The two inductance coils I and II are coupled together by mutual inductance, and each of the variable condensers I and II is so adiusted as to neutralise the reactance of its associated inductance coil, I or II respectively, at the carrier frequency when the other 0011 is open-circuited. The grid of the carrier valve I isbiased at least to anode-current cut-off, and preferably beyond, whereas the grid of the side-band valve I is biased to a point nearl at anode-current cut-oil; both valves are operated so as to be conductive during only part of a cycle. The carrier valve may be conveniently biased by means of a gridleak II and condenser II, as shown in 1'18. la.

It is to be noted that the two coupled inductances I and II together with their associated condensers constitute an impedance-inverting network between the side-band valve 2 and the load I. The arrangement is made such that, in the unmodulated condition, the side-band valve takes no current, and in this condition, the impedance seen looking from the carrier valve I towards the load is substantially equal to the load impedance. In the modulated condition. the sideband valve is operative, and the impedance seen by the side-band valve is arranged to be equal to the matched load of this valve, 50 that the maximum power is supplied to the load.

If the grid'voltage of the carrier valve I is representedby in sin at, then if the carrier valve is assumed to be of low impedance, its anode voltage will approximate to the constant output voltage v sin at, where is the amplification factor of the valve. If the grid voltage of the side-band valve 2 be assumed for the time being to be a pure side-band component. say kv: sin pt cos at, then if this valve is one of high impedance, its anode current can be taken to approximate kl sin pt cos at, where I is the amplitude of the current caused by a grid voltage 02-1. e. where 1/12: is the mutual conductance of the valve I. These two approximations ignore the fact that the impedances of the two valves are finite, and account will be taken later in this specification of the finite impedances of the valves. The anode eurrentinthe side-band aaaaws valve 2 induces across the anode circuit induct ance coil 8 of thecarrier valve I a voltage equal to ductance coil 8,is therefore a k l I k mamm ra and if aim is made equal tov ,V.the load voltage Vll-i-k sit: on any; v which represents a carrier wave the 1pm sin at'moduiated by a modulating wave sin pt to a depth-or modulation k'. -It will be noted that the depth of modulation in the wavefiapplied to the x 7 carrier valve in the caseconsidered is zero, where as in the wave applied to the side-band valve, the depth of modulation maybe regarded as being infinite. g

Ii, as is required, the current in the load 9 of magnitude:

jwM(1+k sin pt)- sin wt Comparing this anode voltage of the side-band valve withits anode current which was above shown to be approximately I (k sin pt) coswt, it will be seen therefore,-that the side-band valve 2 alternately delivers or absorbs power as the sign of the modulation cycle changes. I

In practice, the valves i and 2 are both of finite impedance,- and to maintain the conditions of constant outputvoltage from the carrier valve and: no current in the unmodulated condition in the other, it is clear that the input voltage'supplied to either or both valves must be different from those so far assumed. In theparticular embodiment which has been described, it isconvenientto modify t e input to the side-band valve only, the gridpdrive supplied to the carrier valve beingstill a steady carrier-wave. The output voltage from'thecarrier'valvecan, under these conditions, still be kept very nearlyconstant, and will vary only slightly'withtheourrentintheload. r '1 1 The required griddriv'e iorthev side-band Valve is-deducedas follows: a

1 Theoutput voltage' of the carrierlvalve, I ,is

givenby: v v

' vsn 1 .l; W: 'm", z

where r is theapparent:impedanceqoi the carrler valve. Clearly, pi= shouldbeas small as pos- 'sible compared to theload impedance Ri for high est 'eflioienc'y} The-ElwM. F. wh

n s hibernatewe ,7 current which thecarrier valve anode-circuit inductance coil O of the carrier valve to main a voltage: V

' The current re u res-i pale inductance oil 10 i1 associated with the sid to induce this vol age no: the to side-band; valve is as-beiforez ifme them-m wt The side-band valve, althoughioperated,asla class AB amplifier; can be zr'esa lded as a linear amplifiers with I respect: @to; -;the modulation; en velopef and is therefore: subjectiwi'thf regard to this envelope, to the: law ofysuperposltioni I ts grid voltage mayythereiore be deduced i'rom, a knowledge of its anodeourrent and anode volt: age swing initwo' separate parts,,--- (a)"that ,;re quiredto prevent'thesvalve taking;any anode cur rent "under the influence of an anode gvoltage swing of the form set out above, and (b) that required to produce the' desiredjanode current; in the anodevcircuit coil which, sinceitsinductance is neutralised by its associated condenser, eilectively introduces no impedance; into the circuit. If the apparent impedance ,of the side-band valve isz, and theapparentimagnification fact these parts are: 'i a and I v fits) Th d lvoltas we or Fi m t tice with valves of finite msedanceythas secomes! I I mh s,

which represents a wave corr iprising 'both carrier andtside-band components, but having a depthof modulation diiieringjirom that of the 5 flvoltage'across the load, and, of course, from that appliedtozthe carrier valve.

With given values of components, the peak anode voltage and currentswin'gs', V and I, which are permissible are sno man. as alreadystated, avM is 1 made, equal, to 'V/I, The, load jresis nee R1 is thendetermined by the anode voltage U ing which is' de redzon the ;'side-ban'd valve! si e I this; ,voltagexswing' is equal to:

i giiH pi] ost;

An; advantageof' 'the embodiment oi. he inventionf'described above is that the :carriercvalve can be operated as -'a"class C *continuousswave 7 amplifierj'andvery high efliciency of peration of this part of the apparatus can therefore be obtained; the efllciency is particularly high when the carrier valve is biased beyond anode current cut-oil, as suggested above. The side-band valve takes substantially no current in the absence of modulation, and thus in this condition the emciency of the apparatus as a whole is very high. It is an advantage to employ valves having characteristics which approximate as closely as possible to the ideal rectilinear characteristic with sharp cut-oil, since with such valves, the level of distortion is low.

Because of the finite curvature oi the bottom bends of the characteristics of he valves employed in practice, the side-band valve cannot be biased to draw no current in the unmodulated condition. It is, in fact, biased on to the bottom a bend, and it may be desirable to convert some of the D. C. power which it absorbs into carrier power. This can be done by increasing the carrier component in its grid drive to a value somewhat greater than that calculated, so that the side-band valve contributes to the carrier power supplied to the load. The power supplied by the carrier valve is then reduced slightly below the value calculated, and high efllciency is maintained.

In a modification of the arrangement shown in Fig. l, in which the two valves I and 2 may each be regarded as being connected in series with the load, the two valves and the load may all be arranged in parallel with one another. This modification is shown in Fig. 2. Any known or suitable means are provided in the two arrangements illustrated to supply the required operating anode, filament and grid bias voltages for the valves, and to reduce the production of radio-frequency harmonics. For example, in Fig. 2, the coupled inductances 8, ll constituting the impedance inverting network may be shunted by condensers I4 and L5 as shown, which provide low-impedance paths for harmonic voltages, and also form part of the network.

The required grid voltages may be derived by any known or suitable means from sources of carrier oscillations and modulation frequency oscillations, and since these details are not regarded as forming a primary part of the present invention, they have not been illustrated or described. Further, any suitable means may be employed for coupling any desired number of amplifiers according to the invention in pushpull or parallel with respect to a common load, and for feeding suitable driving voltagesthereto in suitable phase relationship.

In the above description, the embodiment of the invention considered is one in which both driving oscillations include a carrier component,

one of them has no side-band compo- This is one limiting case of the invention,

being that in which both oscillations side-band components, but one hasno I, .canponent as indlcatedin Fig. lb. It will that between these two limits, there is g of choice of the natures-of the two drivo iclllations available for carrying outthe an example has been given of the i 'mdhod by which the two driving oscillations necessary to cause a given desired current to in the load can be determined, and those 1 in the art will readily be able to extend indicated to cover other cases.

Q'hatisclaimedis: A system for delivering modulated carrierpower to a load, which comprises a first divided by the amplification factor of said auxandaseconddischargetubeeachhavingacathode, agrid and an anode,meansfor simplying continuous potential to said anodes, connections for delivering power from the anodes of both of said tubes to said load, an impedance inverting network connected between the anode of one of said tubes and said load so as to present to the anode of said first discharge tube an impedance whose value decreases with increasing power output from said second discharge tube, means for applying to the grids of said tubes driving oscillations displaced substantially in phase and both said driving oscillations including a component of carrier frequency and only one of said driving oscillations including a component of side-band frequency.

2. Apparatus for delivering modulating carrier-wave power to a useful load, which comprises a first and a second discharge tube each having a cathode, a grid, and an anode, means for applying operating potentials to the electrodes of said tubes, an impedance inverting network having an input and an output, connections from the output of said network to one of said tubes and to said useful load, connections from the input of said network to the other of said tubes, and means for applying to the grids of said tubes driving oscillations displaced substantially 90 in phase and one of said driving oscillations having carrier frequency without side-bands and the other of said driving oscillations including having a carrier frequency and a component of sideband frequency.

3. Apparatus according to claim 2, wherein said means for applying operating potentials to the electrodes of said tubes comprises means phase with respect to said main carrier currents is controlled by a modulating voltage, the plate of said auxiliary tube being subject to varying potentials due to the main carrier currents in said load, the method of maintaining substantially uniform operating conditions for said auxiliary tube in the presence of said varying plate potentials, which includes the step of controlling the plate current of said auxiliary tube by a radio frequency bias, said bias having a phase which tends to oppose a change in the plate current of said auxiliary tube due to' said variation,

5. In a modulation system which includes a first vacuum tube for supplying main carrier currents to at load, and an awd r tube supplying auxiliary carrier currents to said load whose phase with respect to said main carrier currents is controlled by a modulating voltage, the plate of said auxiliary tube being subject to varying potentials due to the main carrier currents in said load, the method of maintaining substantially uniform operating conditions for said auxiliary tube which includes the steps or applying to a control electrode of said auxiliary tube a radio frequency bias whose frequency is equal to that of said main carrier, but in phase opposition thereto, and havinga potential at least equal to the peak potential of the radio frequency voltage on the plate of said auxiliary tube, due to the main carrier current in said load,

trodes, means for applying modulating voltages to said grid, means for energizing said grid by a radio frequency voltage whose phase is determined by said modulating voltages whereby the resultant current in said load is determined by said modulating voltages, and means for applyradio frequency bias to said grid to compotentials applied to across said ing a pensate for varying plate said second amplifier due to voltages load.

7. The combination which includes a source of main carrier frequency currents, a first triode amplifier connecting said source to a load, a second triode amplifier having a plate and grid electrodes whose output is delivered to said load and whose plate is afiected by the voltage across said load due to currents from said first amplifier, means including said second amplifier for impressing currents across said load whose phase with respect to currents in said load from said first amplifier is determined by a modulating signal, and means for maintaining constant anode current in said second amplifier in the absence of said modulating signal.

8. The combination which includes a source of main unmodulated carrier frequency currents, an antenna, a first triode amplifier connecting said source to said antenna, a second triode amplifier having plate and grid electrodes whose output is delivered to said antenna and whose plate potential is afiected by the voltage across said antenna, means for applying modulating voltages to said grid, means for energizing the grid of said second triode by a.voltage which produces output currents in phase with currents in said antenna due to said first amplifier when said modulating voltage is of one polarity, and output currents out of phase with currents in said antenna due to said first amplifier when said modulating voltage is of the other polarity, and means for maintaining constant plate current in said second amplifier in the absence of said modulating voltages.

9. A device of the character described in claim 8 which is further characterized in that said means for maintaining constant plate current is a radio frequency bias.

10. The combination with includes a source of main carrier frequency, an antenna, a first triode amplifier and an impedance inverting network connecting said source to said antenna, means for applying modulating voltages to said grid a second triode radio frequency amplifier having grid and anode electrodes, means for impressing the output of said second triode amplifier across said antenna, the phase of the radio frequency output of said second amplifier being determined by said modulating voltages, and means for causing said second triode amplifier to deliver the same amount of radio frequency current to said antenna independently of the potential appearing on said anode due to currents in said antenna from said first amplifier.

11. Apparatus for causing a current corresponding to a modulated carrier oscillation to flow in a load, said apparatus comprising two thermionic valves, each coupled to said load and one of them being coupled to the load through an impedance inverting network, and means for applying to the input ofone valve 8. driving oscillation having no substantial side-band components and means for applying to the input or the other valve a driving oscillation comprising a carrier. frequency and side-band components, said driving oscillations being displaced substantially 90 in phase from each other.

12. Apparatus according to claim 11 in which a valve having no substantial side-band compo- .nents has its control grid biased beyond anode sponding to a modulated carrier oscillation to flow in a load, said apparatus comprising two thermionic valves, one of which is coupled to the load through an impedance inverting network and the other of which is coupled directly to the load, means for applying to the input circuits of said valves driving oscillations of different'character displaced substantially 90 in phase from each other, one of said driving oscillations comprising a carrier frequency without side-band components and the other of said driving oscillations comprising a carrier frequency and side-band components, but principally sideband components.

15. Apparatus for causing a current corresponding to a modulated carrier oscillation to fiow in a load,'saidapparatus comprising two thermionic valves, each coupled to said load and one of them being coupled to the load through an impedance-inverting network, and means for applying to the input of one valve a driving oscillation having a carrier frequency and side-band components and means for applying to the input of the other valve 8. driving oscillation having a side-band component with a different depth than the side-band component on the first valve, said driving oscillations being displaced substantially 90 in phase from each other.

16. Apparatus for causing a current corresponding to a modulated carrier oscillation to flow in a load, said apparatus comprising two thermionic valves, one or which is coupled to the load through an impedance-inverting network and the other of which is coupled directly to the load, means for applying to the input circuits of said valves driving oscillations of different character displaced substantially 90 in phase from each other, each of said driving oscillations comprising a modulated carrier frequency,

the modulation depths of which differ from each other and at least one of said driving oscillations comprising a substantial amount of carrier freput from said second discharge tube, means for applying to the grids of said tubes driving oscillations displaced substantially in phase, one of said driving oscillations including a component of carrier frequency and a component of side-hand frequency, and the other of said oscillations incinuing a component oi side-band irequency of diflerent modulation depth than the first mentioned component of side-band frequency.

l8. Asystem for delivering modulated carrierwavepowertoaloadwhichcomptlsesaflrstand a seeonddischarge tuheeachhaving a cathode,

a grid and an anode. means for supplying continuous potential to said anodm, connections for delivering power from the anodes 0t b'oth of said tubes to said load, an impedance-inverting networkconnectedbetweentheanodeofoneotsaid timesandsaidloadsoastopresenttotheanodeofsaidiirstdischargetubeanimpedance ofside-handtrequeneyandoniyoneotniddriving oscillations including a component of carrier frequency.

19. Apparatus for causing a current corresponding to a modulated carrier oscillation to flaw in a load, said apparatus comprising two thermionicvalvesmneoiwhichisconpiedtothe load through an impedance-inverting network and the other which is coupled directly to'the load,meansfor applyingtotheirinputcircuits driving oscillations comprising components of the said modulated carrier oscillation, the oscillationsofthetvloinputcircuitsbenzdisplaeed substantially-90 in phase, the oscillations ied to one input circuit comprising a preponderance oi carrier component and the oscillations fed to the other input circuit comprising a preponderance of side-band content.

RICHARD LEWIS FORTECUE. 

